Cephalization is an evolutionary trend in animals that, over many generations, the special sense organs and nerve ganglia become concentrated towards the rostral end of the body where the mouth is located, often producing an enlarged head. This is associated with the animal's movement direction and bilateral symmetry, and cephalization of the nervous system led to the formation of a functional centralized brain in three groups of bilaterian animals, namely the arthropods, cephalopod molluscs, and vertebrates (craniates).